1. Field of the Invention
The present invention relates to a super formable high strength thin steel sheet suitable for use in various applications, e.g., automobiles, and a method for manufacturing the thin steel sheet. More particularly, the present invention relates to a thin steel sheet with excellent workability and low-temperature annealing properties as a Ti—Nb-containing steel in which coarse Ti-based or Nb-based precipitates are distributed, and a method for manufacturing the thin steel sheet. The thin steel sheet is subjected to surface treatment and has excellent powdering resistance.
2. Description of Related Art
In recent years, steel sheets for automobiles tend to be shaped into an integral body due to their complicated configurations. A high level of formability is required to satisfy this tendency. At the same time, high strength of steel sheets is also required to reduce the weight of car bodies and to ensure safety of drivers. Accordingly, studies on steel sheets having a high strength and high r-value (Lankford value) are thus being actively undertaken.
Some cold rolled steel sheets for automobiles having a tensile strength of 35 kgf/mm2 grade or more and an r-value of 2.0 or more are disclosed in (1) Japanese Patent Laid-Open No. 5-230541, (2) U.S. Pat. No. 5,360,493 and (3) Korean Patent Laid-Open No. 2002-0047573.
(1) According to Japanese Patent Laid-Open No. 5-230541, a steel sheet for automobiles is manufactured by subjecting a steel slab to lubrication hot rolling at a temperature between the Ar3 transformation point and 500° C., and recrystallizing, cold rolling and continuously annealing the resulting steel slab, the steel slab comprising a Ti—Nb-containing ultra-low carbon steel with 0.2 wt % or less of Al as a deoxidizing element.
(2) According to U.S. Pat. No. 5,360,493, a steel sheet for automobiles is manufactured by subjecting a steel slab to lubrication hot rolling at a temperature between the Ar3 transformation point and 500° C., and recrystallizing, cold rolling and continuously annealing the resulting steel slab, the steel slab comprising a Nb-containing low carbon steel with 0.2 wt % or less of Al as an element for precipitating and fixing AlN.
Subsequent to the above-referenced prior art (1) and (2), however, techniques have been developed in which the steel sheets are manufactured by lubrication rolling in the ferrite zone, such that the steel sheets cannot be manufactured by common hot rolling equipments. In addition, the referenced prior art has disadvantages such that recrystallization annealing must be carried out before cold rolling and continuous annealing temperature is as high as 890° C.
(3) Korean Patent Laid-Open No. 2002-0047573, which was filed by the present inventors, relates to a method for manufacturing a cold rolled steel sheet which comprises a Ti—Nb-containing ultra-low carbon steel with 0.15 wt % or less of Al as a deoxidizing element. The cold rolled steel sheet has a high tensile strength of 40 kgf/mm2 grade or more and a high r-value of 2.0 or more without involving recrystallization of a hot rolled sheet, and at the same time, excellent formability. The method lowers the continuous annealing temperature to 830° C., but there is a need to further lower it.
In the prior art (1), (2) and (3) discussed above, since a galvanizing or galvannealing process is applied to the cold rolled steel sheets, powdering resistance of the galvanized layer is an important factor. However, the cited prior art fails to mention the powdering resistance.